1. Field of the Invention
This invention generally relates to a device for controlling a plurality of converters which are connected in parallel with each other to convert from a DC power source to AC power, and more particularly to a controlling device in which a phase difference between the converters will be maintained at a predetermined value.
2. Description of the Prior Art
In the past a plurality of current control type frequency converters or inverters which are connected in parallel with each other to convert from DC power to AC power, have been employed to control a load, such as an AC electric motor. Each of the converters is provided with a ring counter to transmit signals to controllable semiconductor switching elements, such as thyristors. These ring counters are controlled by control signals from a referernce circuit. Thus, in the case where the AC electric motor is driven by a plurality of parallel connected converters, the generated control pulse is distributed to each converter through a respective ring counter so as to maintain the phase difference between the outputs of each converter at a predetermined value.
However, such a prior art system has a problem in that if a noise signal is added to the distributed pulse, it is impossible to keep the predetermined phase difference as described above, whereby the output of the parallel connected converters includes some distorted waves or some harmonic components. Therefore, it is difficult to continue the operation of the AC electric motor.